Abstract

Dr D. M. D. James writes: I should like to congratulate the authors (Phillips et al. 1976, Journal vol. 132, part 5) on an important and stimulating contribution.

The evidence for collision misfit and dextral transpression along the Iapetus Suture is appealing. Small scale misfits along such a feature which is likely to be a zone 10-100 km wide, are known to produce local regions of compression and extension. Study of the author’s fig. 10 suggests that such phenomena would be likely in N Wales from the L Devonian. I suggest that the emplacement of the Carmel Head ‘nappe’ in Anglesey over L Devonian molasse might be explained in this way, either directly by thrusting from a compressed region of the fault zone or perhaps thrusting aided by gravity gliding from a crustally thickened high area near the suture.

The kinematics indicated in fig. 10 also imply that broadly E-W fracturing in late Ordovician/Silurian times is likely in central Wales. Such fractures could affect sedimentation patterns and I believe there is evidence in support of this (Geol. Mag. 1969,562-82).

The extent of continental crust northwestward of the Irish Sea landmass (to the northern margin of the Leinster Basin in the authors’ reconstruction) is debateable. In N Wales, Wood (1974) implies that the Monian was deposited at least in part on oceanic crust (S.E.P.M. Spec. Publ. No. 19 334-44).

The enormously thick Manx-Skiddaw Slate Series may indicate deposition in a trench or arc-trench gap over crust which, if at all continental,